For an internal observation of a non-transparent tissue with use of an optical microscope, a pretreatment (a clearing treatment for making a subject transparent) using a clearing reagent is performed.
As typical examples of the clearing reagent and the pretreatment method, there have been known (i) a Focus Clear (product name) solution described by Ann-Shyn Chiang in Patent Literature 1 and Non-Patent Literature 1 and (ii) a tissue clearing method described by Hans-Ulrich Dodt et al. in Non-Patent Literature 2. These are both used to make a tissue transparent for an observation of a fluorescent substance existing in the tissue.